This invention relates to an electric motor controlling relay of the type for controlling the starting windings of an electric motor and according to the principles of the present invention may include a much more compact and simplified construction and overall assembly than has heretofore been possible in similar devices, one result being the necessary inclusion of less component parts yet while maintaining a maximum efficiency of intended performance and use. Furthermore, the relay of the present invention may include a unique component formation, positioning and case containment providing efficient air circulation cooling for the typical thermistor incorporated therein and eliminating the usual requirement of auxiliary cooling plates for proper functioning of such thermistor, again adding to the compactness and simplification of the overall assembly. Still in addition, the relay of the present invention may include a unique form of bus bar incorporated rigidly mounted in the assembly normally serving as a part of the usual electrical transmission circuit therethrough, but at the same time, being conveniently accessible and selectively convertible to permit the connection of capacitance into the assembly electrical circuit where such is required.
Various forms of relays and similar devices have heretofore been used for controlling the split-phase starting windings of single-phase electric motors, for instance, the electric drive motors of refrigerators and similar mechanisms. Generally, the functioning of the relay is to maintain the starting windings of the electric motor connected or energized for approximately 3/10 second to 1/2 second during the starting of the motor and at that time, de-energize or cut out the starting windings from the motor circuit which would normally be prior to the motor reaching full running speed. In this manner, the relay automatically controls the provision of additional torque for the electric motor during the starting thereof while automatically removing or eliminating such additional torque just prior to the time that the electric motor will normally reach full speed where such additional torque is not necessary. However, in such electric drive motor starting, there are occasions where the initial load on the motor at the time of such starting is excessive such as can not infrequently occur in refrigeration units and in this case, automatic current overload and/or heat overload controls of the motor immediately interrupt electrical power thereto, the important point being that when such occurs, the particular relay controlling the motor starting windings must be constructed such that it will be ready to again perform its starting winding controlling function by the time that the motor is once again ready to attempt to start.
One of the more commonly used form of relays applied to the foregoing purposes in the present modern times incorporates a thermistor, that is, a resistor making use of a semi-conductor whose electrical resistance varies sharply in a preplanned manner with the temperature thereof. The required characteristics in the present instance will be that the thermistor will initially transmit electrical energy therethrough at virtually full current and will continue to transmit nearly such full current therethrough until the current transmission heats the same to a given temperature, at which time, the thermistor will instantaneously sharply increase its resistance to reduce the current transmission therethrough to a minimal amount. Thus, by providing the thermistor in wafer-like form and transmitting the electrical energy therethrough by abutting contact with metal contact plates against the faces of the thermistor, the thermistor is connected into the electrical circuitry of the motor starting windings so as to transmit the electrical energy at nearly full current for the approximately 3/10 second to 1/2 second while the motor comes up to full speed and then automatically reduce the current to a minimal amount just prior to the motor reaching full speed.
Preceding the development of these more modern thermistor-type relays, the starting windings of the same electric drive motors were normally controlled by a centrifugally regulated relay built directly into the drive motor structure so that it can be seen that the advent of this thermistor-type relay has advantageously decreased the size and complexity of the equivalent electric drive motors. Although the thermistor-type relays require mounting exterior of the electric drive motor housings, the inherent simplicity and compactness of these relays has permitted the mounting thereof in relatively small space areas. This is particularly critical due to the relatively long standing trend toward miniturization and compactness designed into the presently marketed devices. At the same time, such exterior mounting provides the advantage of ready accessibility and simple replacement in the event of malfunctioning or changing demand conditions, such convenience not being present with the prior relays incorporated internally of the electric drive motor.
Despite the limited size and compactness of the presently marketed thermistor-type relays prior to the unique structure of the present invention, however there is still the desirability to even further reduce the size and compactness thereof to permit more convenient exterior mounting, as well as to reduce the electrical circuitry thereof to a minimum number of parts for both cost savings and increased efficiency of operation less subject to malfunction and other failure. For instance, keeping in mind the inherent operation of the thermistor requiring a temperature rise therein generated by current flow therethrough to perform its current flow controlling function as hereinbefore described, the prior thermistor-type relays have made use of metal heat dissipation plates in addition to the normal electrical circuitry contact plates. As stated, whatever relay is being used to control the starting windings of the particular electrical drive motor, if starting difficulties are encountered and the overload controls of the motor are activated, when these overload controls are sufficiently cooled to permit attempted restarting, the particular relay must likewise be ready for such motor starting. In the case of a thermistor-type relay, of course, this means that the thermistor must likewise have sufficiently cooled to be in an initial starting condition and to augment such cooling, the metal heat dissipation plates in conjunction with the normal electrical circuit contact plates have been used.
Furthermore, with the prior thermistor-type relays not only have heat dissipation plates been required in addition to the electrical circuitry contact plates and thermistor itself, but a still further component has been required in order to force the contact plates against the thermistor faces so that proper electrical contact is at all times maintained. In the prior constructions, this has usually taken on the form of some type of spring compression member which abuts and acts against one of the contact plates. With the one contact plate forced and maintained tightly abutting the one thermistor face, the thermistor is thereby forced against the other contact plate and the electrical circuitry is established and maintained. Again, the addition of an added component within the relay which is not only space consuming, but adds to the expense of fabrication and assembly.
Still a further area which warrants efforts of possible improvement in the prior thermistor-type relays presently being marketed has to do with the addition into the relay circuitry of capacitance. Not infrequent situations in matching loads with proper fractional horsepower electric drive motors are encountered where hard starting problems are encountered and the addition of capacitance to the relay electrical circuitry can greatly improve these conditions, usually with the larger motors such as those in the 1/4 horsepower to 1/3 horsepower range. In most cases with the prior thermistor-type relays, such capacitance has been supplied merely by randomly adding the same to the outside electrical circuitry either to or from the particular relay. This obviously can result in problems of proper electrical connection, as well as deter efforts toward the vital compactness and serviceability.